Consumer electrical devices are typically required to be certified against electrical safety standards. The appropriate safety standards vary according to the type of device and adherence to the safety standards is often specified by national regulations. Commonly-required safety certifications for devices using direct current (DC) power specify that the device output voltage(s) remain below a Safety Extra Low Voltage (SELV) level. The requirements for SELV systems are specified in standards such as International Electrotechnical Commission (IEC) 61140 and European Standard (EN) 60335-1. The IEC defines a SELV system as a system in which a DC output voltage cannot generally exceed 60 volts, although limited excursions above this voltage level may be permitted for brief periods.
Lighting circuits typically must meet the requirements of a SELV system. This presents potential problems with light emitting diode (LED) lighting systems, as the power supplies for these systems must often provide voltages close to the 60 volt limit of a SELV-compliant system. For example, the LEDs in such a lighting system may require that an LED driver provide constant current at up to 54 volts, nominally, for proper operation. Due to component tolerances within the lighting system circuit and its driver, variances due to environmental factors (e.g., temperature), etc., a voltage somewhat higher than the nominal 54 volt level may be produced, e.g., 57 volts. This leaves very little margin between the power supply output voltage and the allowed voltage limit of 60 volts, at which level an overvoltage protection circuit needs to disable the output power. A power supply providing an output of 57 volts under normal operation with an LED load is likely to provide a voltage exceeding the 60 volt limit if the LED load is removed or if some other fault condition occurs. Techniques for protecting against such overvoltage events tend to have disadvantages such as higher circuit cost or poor performance.
One technique for accurately protecting against such overvoltage events is to incorporate an overvoltage detection circuit on the secondary side of an isolating transformer in a power supply. This detection circuit can incorporate high-accuracy components (e.g., low-tolerance Zener diodes) in order to detect overvoltage events with great accuracy. A feedback controller on the secondary side could inform a power supply controller on the primary side that an overvoltage event has been detected and that the power supply needs to be disabled. Alternatively, the overvoltage detection could be performed by the primary-side controller, but this would require the use of an opto-coupler or some similar component. Either of these options has the disadvantage that extra circuit components are required leading to a larger footprint requirement for the circuit and higher circuit costs.
A second technique for protecting against overvoltage events is simply to lower the operating output voltage, e.g., from the 54 volts mentioned earlier to 50 volts. This relaxes the accuracy requirement for the overvoltage protection threshold voltage. For example, a nominal threshold of 55 volts could be used to detect an overvoltage event for a power supply driving a nominal output voltage of 50 volts. Should this threshold voltage be moderately inaccurate due to component tolerances and/or the sensing of the output voltage be moderately inaccurate, the output could be kept safely below the 60 volt SELV limit with considerable confidence. This second technique has the disadvantage that it cannot be used to drive circuits that require constant current at a voltage close to 60 volts.
Yet a third technique takes advantage of the aforementioned exception to the SELV requirements, wherein the output voltage may exceed the 60 volt limit by a small margin (e.g., 5 volts) for a brief period of time (e.g., 200 milliseconds). The threshold for detecting an unsafe voltage is set near the 60 volt limit. Should this threshold be reached, the power supply is disabled. This method has the disadvantage that there is no recovery mechanism. Should an overvoltage event occur, e.g., due to a removed load or some fault, the power supply would remain disabled. This means that “hot-plug” functionality would not be supported, and recovery from an overvoltage event would require that the power supply controller be power-cycled or similar.
A power supply controller and method for protecting against overvoltage events at the output of a power supply is desired. This controller and method should require few or no extra circuit components, should be capable of operating close to an overvoltage protection threshold, and should be capable of recovering when an overvoltage event is triggered.